In the semiconductor industry, various manufacturing processes are typically carried out on a substrate (e.g., a semiconductor wafer or workpiece) in order to achieve various results. Processes such as ion implantation, for example, can be performed in order to obtain a particular characteristic on or within the substrate, such as altering the conductivity of a portion of the wafer. A desired impurity material is ionized and accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source or generator for converting a gas or solid material into a plasma. Ions are extracted from the plasma and accelerated to either the desired energy, or to a transport energy. The ion beam is mass analyzed to eliminate undesired ion species, and then, if necessary, accelerated to the desired energy level and directed onto the target workpiece. Most ion implanters use an ion beam that is much smaller in diameter than the wafer and distribute the dose from the ion beam uniformly across the wafer by scanning the beam, moving the wafer mechanically, by a combination of beam scanning and wafer movement, and the like.
In many systems the beam is scanned rapidly in one dimension (fast scan) to form a uniform “ribbon” beam, and then the wafer is scanned slowly through the ribbon in a direction perpendicular to the fast scan. The requirement for excellent surface uniformity means that the beam must be scanned completely off the workpiece in both dimensions. Thus the total implanted area is larger than the workpiece and the beam is not completely utilized. The efficiency of the beam is defined by the ratio of the wafer size to the total implanted area and is always less than 1.
Other inventions have proposed optimized scan waveforms, for example, circular, to decrease the implanted area for electrostatic, magnetic, and mechanical scanners with the overall goal of improving productivity. Typically, in these inventions the scan rate is held constant but the scan width varies with the slow scan. Thus the implanted area is closer in shape to the workpiece, increasing efficiency. However it can be desirable to have a square implanted area (constant scan amplitude) because it allows the beam to always pass over current-measuring devices (dose cups), typically located just beyond the edge of the wafer, and the measured current can then be used in feedback systems to improve uniformity.
Therefore, a need exists for a method for optimizing the scanning of an ion beam, while retaining the desirable constant scan amplitude.